1. Field of the Invention
The invention relates to an automatic transmission mounted on a vehicle. More specifically, it relates to a shift control device in the case that another shift is determined during an operation of a shift (multiplex shift).
2. Description of the Prior Art
An automatic transmission has a multistage shift capability, for example, five forward ratios and one backward ratio. In the automatic transmission, a shift to another ratio may be determined during an operation of a shift to a predetermined ratio. For example, during the operation of a shift from fourth ratio to third ratio (first shift), wherein a first frictional engagement element (C3 clutch) is disengaged and a second frictional engagement element (B4 brake) is engaged, a shift to second ratio, wherein different predetermined frictional engagement elements are disengaged or engaged (C3 clutch or B4 brake is disengaged and B5 brake is engaged), may be determined.
In this case, as described in Japanese Patent Publication (koukoku) Hei 5-50621, a first shift command is output, it is detected that the operation of the shift to a gear ratio set by the shift command is ended. The end of the operation of the shift is determined by detecting that a rotation speed ratio between an input rotation speed and an output rotation speed of the automatic transmission at the gear ratio set by the shift command is kept around 1.0 for a predetermined time. When a second shift command, which is different from the first shift command, is output between the output of the first shift command and the end of the shift to the gear ratio, an operation of a shift based on the second shift command is not performed. That is to say, when the second shift is determined during the operation of the first shift, the operation of the second shift is prohibited until the operation of the first shift is ended. As such, it prevents engine revving or racing and too great a decrease in engine rotation based on a wrong timing of engagement and disengagement of the frictional engagement elements.
In the aforementioned shift control, especially, when the second shift is determined at an early phase of the operation of the first shift, the shift time from the start of the first shift to the end of the second shift becomes long because the second shift is prohibited until the operation of the first shift is ended.
The determination of the second shift during the operation of the first shift is determined as a jump shift, and it may be intended to interrupt the operation of the first shift and to perform the second shift. In this case, a shift shock results from the engine revving. For example, this may occur because of the nature of the first shift when the operation of the first shift is interrupted and the frictional engagement elements operated in the first shift are returned to a state before the first shift.